The invention relates to a focus correcting device for use in a rangefinder camera in which a focussing operation is achieved by an interlocked movement of a rangefinder including its own optics and a focus adjusting mechanism.
As is well recognized, a rangefinder camera includes a rangefinder having its own optics which is utilized to determine a distance to an object being photographed by triangulation, and a focus adjusting mechanism associated with an imaging optics, both of which are connected together so that the object in focus is detected within the field of view of the rangefinder by translating the imaging optics as a distance adjusting ring is turned. Thus, in contradistinction to a single lens reflex camera in which the imaging optics is translated to achieve an in-focus condiition directly, the rangefinder camera requires a mechanism which provides an operational coupling between the rangefinder and the focus adjusting mechanism of the imaging optics.
An exemplary arrangement of a rangefinder and a focus adjusting mechanism of an imaging optics in a conventional rangefinder camera will be described with reference to FIGS. 1 to 3. FIG. 1 shows the optics of a rangefinder, which is formed with a pair of objective windows which are spaced apart by a given distance so that images introduced through the both windows are led into a common field of view in order to determine a distance to an object being photographed by the overlapping relationship of the both images. Referring to FIG. 1, light from an object R being photographed which is incident through one of the objective windows passes through an objective lens 1 and a fixed half mirror 2 to impinge on an eyepiece 3. Light from the object R which is incident on the other window is reflected by a rotatable mirror 4, which is pivotally mounted on a point Q, onto the fixed half mirror 2 and thence reflected to the eyepiece 3. An auxiliary lens 5 is disposed intermediate the half mirror 2 and the rotatable mirror 4.
In the optical arrangement described, when it is focussed to infinity, the rotatable mirror 4 is disposed in parallel relationship with the half mirror 2, with an angle of inclination of 45.degree. with respect to a line joining the points P and Q. In order to determine the distance to the object R which is located at a finite distance, the mirror 4 is rotated until the two images which can be viewed through the eyepiece 3 coincide with each other so that the taking lens is automatically brought to its focussed position, namely, focussed onto the film surface, when the distance to the object R is determined.
In this Figure, the distance d.sub.0 to the object is expressed in terms of the angle .theta. of rotation of the rotatable mirror 4 and the length of the base line l; EQU d.sub.0 .apprxeq.l/2.theta.
FIG. 2 shows an exemplary mechanism which provides an interlocked connection between the rangefinder described and a focus adjusting mechanism of an imaging optics. Specifically, the imaging optics includes a plurality of lenses 6a, 6b, 6c which are carried by a lens barrel 7 which is in turn slidably fitted into a fixed sleeve 9 secured to a camera body 8. The barrel 7 fixedly carries a distance adjusting pin 10 which fits in an inclined slot 9a (see FIG. 3) formed in the fixed sleeve 9, so that as a distance adjusting ring, not shown, is rotated, the pin 10 angularly moves around the optical axis of the lenses to cause a movement of the lenses 6a to 6c in the direction of the optical axis in accordance with the constraint defined by the slot 9a. An apertured diaphragm 11 is disposed within the barrel 7, and its position is controlled by a diaphragm setting member, not shown, to provide a desired diaphragm aperture.
An interlocking pin 12 is secured to the rear end of the lens barrel 7, and has its free end 12a disposed in abutment against one end 13a of a connecting lever 13 which is pivotally mounted on a pin 14. A coiled spring 15 normally urges the lever 13 to rotate counter-clockwise about the pin 14. Consequently, the end 13a is maintained in abutment against the free end 12a of the pin 12. A pin 16 is fixedly mounted on the other end of the connecting lever 13, and is maintained in abutment against the edge 17a of a rotatable arm 17 adjacent to one end thereof. A hollow shaft 18 is fixedly mounted in the other end of the arm 17, and is tightly fitted over a support shaft 19. A coiled spring 20 urges the arm 17 to rotate clockwise about the shaft 19, whereby the edge 17a is maintained in abutment against the pin 16. The rotatable mirror 4 which forms part of the rangefinder optics is fixedly mounted on the hollow shaft 18 which is adapted to rotate integrally with the arm 17.
In the conventional mechanism thus constructed, when a distance adjusting ring, not shown, is turned for the purpose of focussing, the barrel 7 which carries the lenses 6a-6c moves either forwardly or rearwardly in the direction of the optical axis, whereby the interlocking pin 12 which is connected therewith moves in the direction of the optical axis, as indicated by a double-headed arrow A. In response thereto, the connecting lever 13 angularly moves about the pin 14 to cause an angular movement of the rotatable arm 17, which is maintained in abutment against the pin 16 on the lever 13, about the shaft 19. The hollow shaft 18 which is integral with the arm 17 fixedly carries the rotatable mirror 4, and hence the angle through which the arm 17 moves directly represents the angle of rotation of the rotatable mirror 4. Assuming that the connecting pin 12 moves forwardly, as viewed in FIG. 2, to thereby carry the lenses 6a-6c forwardly therewith, the connecting lever 13 and the rotatable arm 17 turn counter-clockwise, as indicated by arrows B, C. Conversely, when the interlocking pin 12 moves rearwardly, the lever 13 and the arm 17 turn clockwise. Since the connecting lever 13 and the rotatable arm 17 are urged by respective springs 15, 20 to follow the movement of the interlocking pin 12 in a precise manner, it is assured that the fore-and-aft movement of the lenses 6a-6c is positively transmitted to the rotatable mirror 4.
As an alternative, a helicoid system may be employed to move the barrel 7. However, whatever system is employed to move it, a mechanism which provides an interlocking connection between the rangefinder and the focus adjusting mechanism of the imaging optics is constructed to cause an angular movement of the rotatable mirror in response to a movement of the taking lenses, in the similar manner as described above in connection with the illustrated interlocking mechanism.
It will be appreciated that a taking lens which constitutes the imaging optics contains a spherical aberration, which causes an optimum focal point to be displaced in response to a change in the F-number (aperture ratio) corresponding to the choice of a diaphragm setting. In addition, the focal point is also displaced in accordance with a distance to an object being photographed.
Considering this more specifically, FIGS. 4 to 9 graphically show the relationship between a magnification taken on the ordinate (the distance to an object being photographed) and the focal point taken on the abscissa, with varying values of the aperture ratio of the imaging optics chosen as a parameter. A characteristic curve a shown in solid line represents a displacement of the focal point when the diaphragm is open while a characteristic curve b shown in broken lines represents a displacement of the focal point when the diaphragm aperture is partly reduced, for example, reduced by two steps.
The focal point of the imaging optics is displaced in accordance with a variation in the diaphragm value. Depending on the magnification, the displacement of the focal point may change linearly as shown in FIG. 4. Alternatively, it may change non-linearly as indicated in FIGS. 5 to 7. As a further alternative, the displacement of the focal point with a change in the diaphragm value may remain constant regardless of the magnitude of the magnification, as illustrated in FIGS. 8 and 9. In FIG. 4, chevron-shaped curves c, d represent the lens performance, exhibiting the best performance at the peak. Where the diaphragm is changed by two steps, the depth of focus increases to substantially twice its original value.
In this manner, in the imaging optics, the focal point is displaced in various ways depending on the diaphragm value or F-number principally because of the spherical aberration of the imaging optics.
In a conventional imaging optics, the optical system is designed so that the displacement of the focal point is limited within the depth of field and the depth of focus.
However, in a conventional rangefinder camera which has been described above in connection with FIGS. 1 to 3 and in which the rangefinder is interlocked with the focus adjusting mechanism of the imaging optics, the imaging optics is designed to provide the best focus exactly upon the film surface when the diaphragm aperture is open. If the aperture is reduced, the taking lens may not be accurately focussed upon the film surface and thus is defocussed strictly speaking, even though doubled images coincide with each other in the rangefinder.
Such defocussed condition may be corrected by causing the taking lens to move slightly in the direction of the optical axis. However, when so corrected, since the rangefinder is precisely interlocked with the imaging optics, doubled images within the rangefinder cannot be brought into coincidence with each other, thus presenting a difficulty.
Thus it will be seen that it has been a problem in the prior art of optical instruments such as cameras to provide means which achieves a precise optical focussing of an imaging optics which is subject to an increased displacement of the focal point in response to a reduced diaphragm aperture. In the case of an optical system which is provided with the rangefinder, various attempts and studies have been made including an improvement of an interlocking mechanism between a rangefinder and a focussing movement of a taking lens, a remedy to compensate for a displacement of the focal point in response to a change in the F-number. In particular, if it is possible to achieve an accurate focussing through a displacement of the focal point in accordance with the F-number, a defocussing condition which results from a change in the diaphragm value can be avoided, allowing the achievement of an excellent lens performance to be expected.